This invention relates to a method for operating fluid machines in spinning-in-air mode.
In this specification, fluid machines include pumps, turbines and reversible pump turbines. Generally, in fluid machines directly connected to a generator-motor, the runner may be rotated by the generator-motor in compressed air atmosphere in the runner chamber after the liquid level in the runner chamber is moved downwardly. Such pattern of operation, which is generally referred to as a spinning-in-air mode operation, is practiced also for producing a so-called standby condition which permits immediate switch-over to a power-generating or pumping-up operation for producing initial torque of the generator-motor at starting of pump-operation or for improving power factor of the transmission system.
In one method known in the art for practicing a spinning-in-air mode operation, as exemplified by U.S. Pat. No. 4,179,237, main machinery consisting of the reversible pump turbine and motor-generator is shut down and the liquid level in the runner chamber is moved downwardly before a starting electric motor is actuated to rotate the runner and increase its number of revolutions until the rated rotational speed is attained. This method is similar to the ordinary method for starting pump-operation. In a second method known in the art, the inlet valve is opened to fill the casing with water, the guide vane is opened to a predetermined degree of opening to rotate the runner, and the number of revolutions of the runner is increased until the runner speed reaches its rated speed. After the fluid machine and the generator-motor are connected in parallel with the transmission system, the guide vane is fully closed and compressed air is fed to the runner chamber to move the liquid level therein downwardly below the runner to permit the latter to rotate in compressed air atmosphere in the runner chamber. In the one method, the liquid level can be moved downwardly relatively easily because the main machinery is in-operative. However, this one method has the disadvantages that there is a need to mount the starting electric motor and that it takes time for the runner to attain its rated speed of rotation. Therefore, it is customary to use the second method for practicing spinning-in-air mode operation of a fluid machine. The present invention relates to improvements in the second method.
In this second method, proposals have been made to effect control of thrust acting on the runner by means of a leak-water exhaust pipe, a leak-water exhaust valve and an air exhaust valve which influence the pressure in the runner crown chamber and the runner band chamber formed on the upper side and the lower side respectively of the runner. A method and apparatus for practicing this second method of operation is disclosed in U.S. Pat. No. 3,890,059, for example.
When a fluid machine is to be operated in spinning-in-air mode according to the second method, the guide vane is fully closed after the rotational speed of the runner has reached its rated speed and the main machinery is connected in parallel with the transmission system, and then compressed air is fed to the runner chamber to move the liquid level therein downwardly below the runner while at the same time closing of the inlet valve is begun, as aforesaid. The water present in the runner chamber would be readily subjected to the influences of centrifugal forces produced by the blades of the runner so that a certain amount of water would form an air curtain in the outer peripheral portion of the runner without being moved downwardly to a level below the runner in the runner chamber. Also, water leaking through the side gap of the guide vane and the cooling water supplied to the runner sealing sections would be scattered to the outer peripheral portion of the runner, thereby increasing torque of the runner rotating in compressed air atmosphere in the runner chamber.
Therefore, in the aforesaid second method for practicing spinning-in-air mode operation of a fluid machine, an attempt has been made to reduce torque of the runner rotating in compressed air atmosphere in the runner chamber by discharging the residual water through the leak-water exhaust pipe mounted either at the lower cover or the upper cover or between the runner and the guide vane by opening the leak-water exhaust valve mounted in the leak-water exhaust pipe, after the guide vane is fully closed and the liquid level in the runner chamber is moved downwardly below the runner.
However, when the volume of water in the runner chamber is large and the pressure thereof is high, the volume of water discharged through the leak-water exhaust pipe into the draft pipe per unit time would be great because the pressure differential between the interior of the draft pipe and the water in the runners chamber is large. Thus the liquid level moved downwardly by compressed air must cover a large distance in its downward movement, and consequently it might become impossible to continue the spinning-in-air mode operation. This problem may be solved by moving the open end of the leak-water exhaust pipe on the draft pipe side to a lower level to prevent the liquid level in the runner chamber from being influenced by the water discharged through the leak-water exhaust pipe, or by moving the liquid level to a position much lower than the position to which the liquid level has hitherto been moved downwardly by the compressed air. However, such solutions to the problem would entail an increase in the length of piping or an increase in the size of liquid-level pushing-down equipment because an increased volume of water must be handled in moving the liquid level downwardly in the runner chamber.